1. Field of Invention
This invention relates, in general, to resistance spot welding equipment and, more specifically, to resistance spot welding electrodes for coated metals and a method for welding coated metals.
2. Description of Prior Art
Resistance spot welding is a process used to join two or more pieces of metal together by electrically inducing localized fusion of the metal. Usually, the two or more pieces of metal are thin sheets. However, the process has been substantially useful for joining thicker sheets of metal and coated sheets of metal, such as zinc coated metals or galvanized steel.
The spot welding process generally involves clamping the metal pieces or workpieces to be joined together between two axially aligned electrodes and applying high pressure thereto. Then, the two electrodes have a high electrical current passed between them. The electric current, passing between the electrodes, passes through the metal pieces. Any resistance point or location in the path of the current becomes heated. The heat is generated proportionally at each resistance point throughout the circuit in accordance with the formula Q(heat)=I.sup.2 (current) R (resistance). This process is complex, involving the interaction of electrical, thermal, mechanical, metallurgical, and surface phenomena.
There are generally two types of locations where resistance occurs in the path of the current: (1) at the interface of the electrode and the workpiece and, (2) at the interface of the metal pieces. This is true for all types of resistance spot welding and for all types of metals. When the metal becomes hot at the interface of the metal pieces, they melt, merging together at the localized spot and, upon cooling, form a spot weld or nugget. Great effort is made to keep the electrodes cool during the welding process and therefore keeping the electrode-workpiece interfaces cool and below the melting point of the metal. This prevents the electrode and workpiece from welding together.
There are several factors involved in obtaining an acceptable and quality spot weld: The type of metal to be welded; the type of electrodes to be used; the amount of clamping pressure applied by the electrodes; the amount of electrical current needed to accomplish the weld and the time to accomplish the weld. Of all the above factors, the electrodes have the most impact upon the others. Resistance welding electrodes are normally made of copper alloy and are usually cooled by water. The electrodes serve three essential functions in the welding process: (1) their low electrical resistance provides a conduit to carry a high electrical current to a workpiece without significant heating (Joule) losses; (2) their high thermal conductivity provides a method for conducting heat from the workpiece by controlling the cool-down process, thereby the weld nugget formation; and (3) their concentrated force on the outer surface of the workpieces to be joined properly seats the workpieces to establish a good interface and good electrical contact before electrical current is applied.
However, electrodes generally have a tendency to lose shape (mushroom) during repeated weld cycles. The repeated mechanical pressure cycles can cause mechanical fatigue which leads to electrode distortion. Mushrooming interferes with the ability of the electrode to focus or localize the electric current and interferes with producing a good quality weld. Until the electrode becomes almost useless, adjustments can be made in the other factors to accommodate the electrodes. For example, weld quality can be restored with a current boost, "stepping". A reshaping (dressing) of the electrode to restore the original area at the contact face is usually necessary when all other adjustments fail.
The total electrical resistance of a welding system is identified as the bulk material resistances of the electrode-workpiece interfaces and the surface contact resistance at the interface of the workpieces. These interface resistances are due to surface films, oxides, and asperities. A high electrode pressure, which produces a localized compressive interfacial stress, mechanically breaks down the surface conditions, thereby providing good interfacial electrical and thermal conductance.
The overall resistance of the copper electrodes and the metal to be joined is small. Therefore, a large electrical current is needed to produce a high heating effect in the workpiece. This causes a voltage drop to occur within the workpiece, since the resistivity of the copper electrodes is lower than the metal to be joined. The highest level of heat is therefore at the interface of the workpieces, which produces a temperature distribution to both the workpieces and the electrodes.
If the high current is maintained for a sufficient length of time, localized melting will occur at the interface between the two workpieces and spread to produce the weld nugget. This change from solid to liquid affects a dramatic change in material properties. During the weld cycle, the electrode pressure is maintained to offset the high internal thermal expansion and, thereby, contain the molten pool of metal at the interface of the workpieces. This prevents liquid metal expulsion.
The electrode pressure also helps to maintain proper electrical and thermal contact until the formation of the weld nugget is completed. The weld cycle is terminated by switching the current off while maintaining the electrode pressure. The final stage of this process is the hold cycle, which establishes the metallurgical quality of the weld nugget. During the hold cycle, the nugget cools and contracts. Mechanical pressure is essential to provide the necessary forging pressure to obtain a good metallurgical structure and to prevent the formation of shrinkage voids in the nugget. The hold time is important since it establishes the rate of cooling.
Until the introduction of coated metals, the problems with electrodes were easily resolved by adjusting the other factors and dressing the electrode. However, the use of coated metals, such as galvanized steel, has caused the electrodes to deform faster and/or alloy with the coating.
Alloying occurs when the coating on the metal sticks to the electrode. Zinc (as well as most other coating materials generally) is more electrically conductive than steel. Therefore, the addition of a layer of zinc (or any highly electro-negative coating material) to the workpiece surfaces reduces the overall system contact resistance. This necessitates a corresponding increase in the electrical level of current in order to produce a weld. The result is higher temperatures at the electrode-workpiece interface. These elevated temperatures cause an additional problem when welding coated steels. The coating material has a tendency to alloy with, and stick to, the face of the copper electrodes. The resultant coating "picks up" on the electrode face and, thus, increases its resistance causing further localized heating and accelerated electrode wear as well as a higher incidence of poor quality welds.
The industry has tried to solve the problems by developing creative weld schedules (adjusting the other factors) or creating new electrode alloys. This has resulted in limited success.
Weld schedule manipulation through slope control and multiple-pulse capability have become common methods used on weld controllers to accommodate this procedure. Special heat resistant copper alloys have been developed and used to reduce the tendency of electrodes to soften under high heat conditions. Electrodes of this type are now in common use. They are coupled with high energy weld schedules that establish electrode/steel substrate contact by burning through and/or vaporizing the coating material prior to producing a weld. In all cases, the excessive application of current requires the use of inordinately high levels of energy, and severely compromises the intended function of the protective coating. Quite often the high application of current results in severe expulsion of molten steel from the weld zone with a corresponding deterioration of the structural integrity of the weld, and a negative impact on operator health and safety.
The methods developed to avoid problems associated with the welding of coated metals are generally counter-productive. Several problems still exist for welding coated metals; For example:
(a) coated metals require a high current surge (up-slope) at the beginning of the welding time to melt and break through the resistance of the coating; PA1 (b) up-sloping melts away the coating protection and allows the electrode to penetrate the coating and contact the substrate; PA1 (c) up-sloping increase the alloying of the electrode and the coating; PA1 (d) up-sloping causes the electrode to become hot and to mushroom and deform faster and increase electrode servicing; PA1 (e) all of the above may cause a poor quality weld; and PA1 (f) excessive use of energy. PA1 (a) applying a clamping pressure upon at least two coated workpieces with axially opposing heat stabilizing electrodes such that workpieces form to each other to eliminate any air spaces at a welding location of an interface of the workpieces; PA1 (b) applying a cooling liquid into a cavity of an electrode such that the temperature at the electrode-workpiece interface remains stabilized below the melting point of the coating material on the surface of the workpiece during the weld cycle; PA1 (c) applying an electrical current through the electrodes sufficient to create a temperature which, in cooperation with the cooling liquid, is below the melting point of the coating at the interface of the electrode and the associated workpiece but is at least the melting point of the substrate of workpiece at the interface of the workpieces, thereby forming a molten weld nugget at the interface; PA1 (d) discontinuing the electric current while continuing to apply the clamping pressure and the cooling liquid to the electrode to cool the workpiece such that the weld nugget has a balanced cooing and solidification; and PA1 (e) thereafter disengaging the clamping pressure from the cooled workpiece.
One example of an electrode to address the coated metal problem is disclosed in U.S. Pat. No. 4,588,870. The invention disclosed therein is an aluminum and copper alloy electrode that has an internal cavity with a portion that partially extends into the tip of the electrode. This partial extension of the cavity apparently provides for additional electrode cooling. The extension, although providing for some additional cooling, does not provide a significant increase in the life of the electrode or a significant energy or cost savings.
Thus, it would be desirable to provide a spot welding electrode and a method of using the electrode for coated metals which has overcome the previous problems, has increased its useful life, and provides a significant energy and cost savings.